A hard disk drive typically includes one or more magnetic disks rotatably mounted in association with a spindle and one or more actuators for positioning a transducer or head relative to data tracks on the surface of each disk. The actuator typically comprises a pivotable base plate, a load beam or suspension arm affixed to the base plate, a flexure at the distal end of the load beam and a slider and transducer or a head assembly affixed to the flexure. A voice coil motor induces movement of the actuator to position the head relative to the disk surface. Typically, the voice coil motor and servo system provide both gross positioning of the head, i.e., track to track positioning, and fine positioning, i.e., track following. Recently, piezoelectric elements have been incorporated into actuators to provide fine positioning of the head assembly for track following purposes, rather than only relying upon the voice coil motor for fine positioning.
With the emphasis on making hard drives smaller for numerous applications including portable computers, magnetic disks are not only becoming smaller, but data tracks are becoming increasingly more densely positioned on the disks and the tracks themselves are becoming physically narrower. As a result, maintaining the transducer or head in an accurate track following position for purposes of reading and writing is becoming more complex. To accommodate increasingly finer adjustments in the position of the magnetic head, dual positionable actuators have been introduced. In a first mode, a voice coil motor will move the actuator from track to track. In a second mode, the voice coil motor will provide fine positioning of the distal end and head assembly, together with a fine actuation element positioned on the actuator. Piezoelectric elements are commonly used as fine actuation elements. By supplying a current to a piezoelectric element, the piezoelectric element can expand and contract in a controlled manner to adjust the distal end of the actuator arm and thereby accurately maintain position of the head relative to the disk surface.
With reference to FIG. 1, an example of the end portion of a dual actuator 10 is illustrated. As can be seen, the forward end of the dual actuator 10 comprises a suspension load beam 12 which includes a secured end portion 14 attached to an actuator arm 15 and a suspended end portion 16 opposite the secured end portion. More particularly, a swage plate 18 with a cylindrical portion or bushing 20 is positioned in a complementary hole in the distal end of an actuator arm 15 and the adjacent metal surfaces are swaged or joined together. A pair of arm segments 22, 24 extend between the main body of the secured end portion 14 of the suspension load beam 12 and the forward portion 16. At the forward end 16, the suspension load beam 12 supports a flexure member 26, which in turn supports a slider, including a magnetic head that reads digital information from and writes digital information to the disk (not shown).
A piezoelectric element 32 comprising two piezoelectric crystals 34, 36 having generally the same physical configuration and properties, are affixed to the arm segments 22, 24. The piezoelectric crystals 34, 36 are commonly positioned on the arm segments 22, 24 with opposed polarizations. A wire 38 electrically connects each of the piezoelectric crystals 34, 36 to a driver on the printed circuit board of the hard drive (not shown). While two piezoelectric crystals are illustrated, the piezoelectric element 32 may also have a single piezoelectric crystal affixed to one of the arm segments 22, 24. Likewise, more than two piezoelectric crystals may be affixed to the arm segments 22, 24.
The driver is generally either a voltage control driver, or a charge control driver. The piezoelectric crystals 34, 36 can be modeled as a capacitor (Cpiezo). A voltage control driver controls the voltage (Vpiezo) across each piezoelectric crystal 34, 36, and a charge control driver controls the amount of charge (Qpiezo) driven into and out of each piezoelectric crystal 34, 36. The two control parameters are related by the equation (Qpiezo=Cpiezo*Vpiezo).
A charge control driver can be advantageous because the charge gain of a piezoelectric crystal (i.e., the output position versus input charge) is relatively consistent over temperature and part variation, compared to the voltage gain (i.e., the output position vs. input voltage) of a voltage control driver. Unfortunately, a charge control driver has several disadvantages. One such disadvantage is that a charge control driver requires relatively complex circuitry in order to be able to drive a specific amount of charge into and out of the piezoelectric device, as compared to the circuitry required for a voltage control driver. This relatively complex circuitry is also expensive to design and manufacture, as compared to a voltage control driver. Accordingly, it would be advantageous to have a piezoelectric gain which is consistent over temperature and part variation, and also a driver that is relatively simple and inexpensive to design and manufacture.
Another disadvantage of a charge control driver is that the dynamic range of the output driver is dependent on the number of piezoelectric elements being driven. The driven charge is distributed between the elements (i.e., two elements need twice as much charge to cause the same displacement as a single element). This sensitivity to the number of elements included in the drive requires the ability to select a discrete dynamic range setting within the charge control driver to match the number of piezoelectric elements. Thus, if a disk drive has a number of actuator arms, a driver needs to be set to have a dynamic range to accommodate all of the piezoelectric elements present in the drive.
As will be understood, numerous configurations exist for the number of disks, and thus actuator arms, present in disk drives. In order to help reduce costs, it is advantageous to use as many common parts as possible for all disk drive configurations. Also, as will be understood by those of skill in the art, disk drive operations are generally controlled using firmware present within the disk drive. It is also advantageous to use common firmware for different disk drive configurations, in order to simplify manufacturing and reduce development costs associated with developing firmware. Accordingly, it would be advantageous to have a driver which is able to drive a variable number of piezoelectric elements, depending upon the particular requirement for a disk drive. For example, a driver may be needed for a disk drive with a single disk, and thus a single actuator arm. A driver may also be needed for a disk drive with four disks, and thus four actuator arms. Using a common driver, and thus common circuit board for the different disk drives would be advantageous for the purposes of making the manufacturing process less complex by having fewer differences in component parts between the two disk drives. Furthermore, common component parts would also allow for a common inventory between the disk drives, as well as a reduction in development costs which would be required to design the different components. Accordingly, it would be advantageous to have a common driver and common firmware which are able to drive a variable number of piezoelectric elements, depending upon the configuration of disk drive in which the driver is used.
The voltage control driver can solve many of the above-mentioned disadvantages associated with a charge control driver. First, a voltage control driver is a less complex driver implementation than a charge control driver, and therefore less expensive. The voltage control driver is also not dependent upon the number of piezoelectric elements being driven, and is thus more suitable for use as a common component which may be used in disk drives having different numbers of piezoelectric elements. The firmware used to control a voltage control driver thus may also be common for disk drives having different numbers of piezoelectric elements. However, the voltage control driver also has several disadvantages.
One disadvantage of the voltage control driver is that the voltage gain (i.e., output position versus voltage) is more sensitive to temperature and part variation than a charge control driver. Temperature variation within the disk drive, and thus within the piezoelectric element(s), can result in a variation in the apparent capacitance. This variation can result in positioning errors in the transducer is they are not properly compensated. In addition to temperature variation, the variation in parts can also result in a variation in the apparent capacitance. As an important consideration in the manufacture of hard disk drives is the cost of component parts, it is often advantageous to have component parts which have a relatively large tolerance. If a relatively small tolerance is required for a component part, the unit cost of that component is generally more than the unit cost of a component having a relatively large tolerance. Therefore, it would be advantageous to have a relatively inexpensive driver which is able to compensate for variation in the piezoelectric elements.
Additionally, when designing a driver for a piezoelectric element, a proper voltage gain is needed for a voltage control driver, and a proper dynamic range setting is needed for a charge control driver. As mentioned above, it would be advantageous to use common components. Furthermore, as different applications may be better suited for a charge control or voltage control driver, it would also be advantageous to be able to have a circuit which is able to compensate for either a charge control driver, or a voltage control driver.
Accordingly, it would be advantageous to have a dual actuator disk drive which (1) is relatively inexpensive; (2) is able to support a variable number of piezoelectric elements without firmware or driver modifications; (3) is able to accurately compensate for variations (e.g., temperature variations) which may be present in the piezoelectric element(s); and (4) has a calibration circuit which is able to compensate for both charge and voltage control drivers.